The stratum corneum is chiefly responsible for the barrier properties of skin. Thus, it is this layer that presents the greatest barrier to transdermal flux of drugs or other molecules into the body and of analytes out of the body. The stratum corneum, the outer horny layer of the skin, is a complex structure of compact keratinized cell remnants separated by lipid domains. Compared to the oral or gastric mucosa, the stratum corneum is much less permeable to molecules either external or internal to the body. The stratum corneum is formed from keratinocytes, which comprise the majority of epidermal cells that lose their nuclei and become corneocytes. These dead cells comprise the stratum corneum, which has a thickness of only about 10-30 microns and protects the body from invasion by exogenous substances and the outward migration of endogenous fluids and dissolved molecules. The stratum corneum is continuously renewed by shedding of corneum cells during desquamination and the formation of new corneum cells by the keratinization process. Historically, the majority of drugs have been delivered orally or by injection. However, neither the oral or injection route is well-suited for continual delivery of drugs over an extended period of time. Further, the injection method of administration is inconvenient and
uncomfortable; additionally, needles continue to pose a hazard after their use. Therefore, transdermal drug delivery to the body has been a popular and efficacious method for delivering a limited number of permeants into an organism.
To enhance transdermal drug delivery, there are known methods for increasing the permeability of the skin to drugs. For example, U.S. Pat. No. 5,885,211 is directed to thermal microporation techniques and devices to form one or more micropores in a biological membrane and methods for selectively enhancing outward flux of analytes from the body or the delivery of drugs into the body. PCT WO 00/03758, published Jan. 27, 2000, is directed to methods and apparatus for forming artificial openings in a selected area of a biological membrane using a pyrotechnic element that, when triggered, explodes in a controlled fashion so that the micro-explosion produces the artificial opening in the biological membrane to a desired depth and diameter. PCT WO98/29134, published Jul. 9, 1998 discloses a method of enhancing the permeability of a biological membrane, such as the skin of an animal, using microporation and an enhancer such as a sonic, electromagnetic, mechanical, thermal energy or chemical enhancer. Methods and apparatus for delivery or monitoring using microporation also are described in PCT WO 99/44637, published Sep. 10, 1999; U.S. Pat. No. 6,022,316; PCT WO 99/44508, published Sep. 10, 1999; PCT WO 99/44507, published Sep. 10, 1999; PCT WO 99/44638, published Sep. 10, 1999; PCT WO 00/04832, published Feb. 3, 2000; PCT WO 00/04821, published Feb. 3, 2000; and PCT WO 00/15102, published Mar. 23, 2000. Applicants would note that all publications, patents and patent applications referred to herein, such as those above, are incorporated herein by reference in their entirety.
There remains a need for improved methods and devices for transdermal delivery of permeants such as, for example, drugs, bio-active compositions, and the like.